Competitive mass production of dynamoelectric machines in the form of electric motors such as those used in household appliances and other machines requires in the design and manufacture of the motor a simultaneous emphasis on speed and simplicity of manufacture, and the precision of the final motor construction. Moreover, any design or manufacturing process must not add costs out of proportion to the savings achieved through higher production. Thus, the present invention pertains to a motor which incorporates design features optimized for speed of manufacture and precision of the final product.
It is well established that the formation of the stator core of an electric motor may be most efficiently carried out by forming the core from a stack of laminations stamped from a sheet of highly magnetically permeable material. The laminations are frequently square because this shape wastes less of the sheet material from which the laminations are stamped. Each lamination is stamped with a central opening and radially extending slots which typically open into the central opening. The central openings of the stator laminations in the stack form the bore of the stator core and the slots define the teeth which extend the length of the stator bore and receive the wire windings of the motor. The slots are stamped symmetrically about the center of the central opening, leaving substantially equal amounts of material along each of the four edges of the lamination. Thus, the amount of magnetic flux which can be carried by the stator core is substantially the same along all four of its sides.
It is important that the stator bore be round and straight so that the rotor may freely rotate in the stator core bore while maintaining only a minimal separation between the rotor and the stator core. The straightness of the bore is adversely affected by the inherent presence of variations in thickness (called "gamma" variation) of the rolled sheet material from which the laminations are stamped, so that each lamination is not truly flat. If the laminations are stacked one on top of the other in the same orientation as when each lamination was stamped on the sheet material, the gamma variations will tend to add together rather than cancel out. Thus, the stator bore formed may be substantially curved and unsuitable for mating with the rotor in such a way which will permit the rotor to freely rotate in the stator bore. Punching the central openings of the laminations from the sheet material relieves certain stresses in the material, which tends to cause the material to elastically deform from the round shape struck by the punch, to an elliptical shape. Further deviations from round may be introduced by thermal stress as the stator core is annealed. Again, if the laminations are stacked together in such a way as to add the deviations from round, a bore which is too elliptical to receive the rotor may be produced. In a square lamination having substantially equal amounts of material remaining after punching on all four sides, deformations causing deviation from round can be expected to occur approximately equally along two perpendicular axes lying in the plane of the lamination. Accordingly, it is preferred to rotate each lamination 90.degree. relative to the adjacent lamination in the stack so that gamma variations and deviations from round in the laminations tend to cancel each other out.
However, in the past 90.degree. rotation of each lamination relative to the adjacent lamination in the stack has not been practical when constructing stator cores for certain two speed electric motors having two windings which have different numbers of poles. In a two speed motor having a four pole winding and a six pole winding, some of the turns of wire forming the poles must be placed in the same stator slots. In order to provide enough room, the slots where the windings will overlap must be deeper. This requirement introduces asymmetry in the arrangement of slots about the center of the central opening of each lamination, and reduces the amount of material on two of the sides of the lamination relative to the other sides. Equalizing the amount of material on all four sides may be accomplished by elongating the two sides having the deepest slots. However, the combination of the asymmetry of the slot arrangement and the rectangular shape of the lamination makes it impossible to rotate the laminations 90.degree. relative to the adjacent lamination when stacking. The best that can be done presently is to rotate the laminations 180.degree., which does not permit cancellation of manufacturing tolerances as efficiently as 90.degree. rotation, and thus adversely affects the roundness and straightness of the bore.
It is well known that in order to decouple stator slot order harmonics the rotor bars in the squirrel cage rotor of an induction motor should be skewed. Typically, skewing is accomplished by turning the rotor laminations making up the rotor slightly with respect to each other so that the passages formed by overlapping slots of the rotor laminations are generally helical in shape. Helical skewing can be carried out by hand using a jig, or automatically by machine. In the former instance, substantial labor costs are added to the production of the rotor, and in the latter instance it is difficult to reliably automate the delicate operation of turning the rotor laminations slightly relative to each other. Further, the helical passages have a stair-step configuration which can produce undesirable turbulence in the molten material poured into the passages to form the rotor bars. Significant savings can be realized by implementation of a "straight" skew, in which the rotor bar passage consists of two smooth, straight passages which overlap, but are skewed. The skewed passage is typically formed by making the rotor slots asymmetrical about a radial line of the rotor lamination, with the slots in one half of the stack of laminations forming the rotor being the mirror image of the slots in the other half. Although decoupling slot harmonics by using two straight passages which are skewed relative to one another is known, there is presently a need for such a straight skew which delivers better motor performance for single phase motors.
Once the rotor and stator have been constructed, it is necessary to assure that the rotor will be aligned with the stator core bore when the rotor is inserted into the bore. The rotor shaft is typically supported for free rotation at its ends in central openings in metal end frames which are connected to the stator core. Tolerances inherent in the formation of the central openings in the end frames and the stator core bore, and the absence of accurate location mechanism for the end frames on the stator core result in many rotor/stator core assemblies being out of alignment. Present practice calls for the introduction of shims in the central openings where the rotor shaft is received to bring the rotor and stator core into alignment. This procedure permits only a relatively coarse adjustment, and requires time and extra labor to accomplish.
The manufacturing step of mounting the rotor shaft on the end frames also presently requires significant labor and time to accomplish. The ends of the rotor shaft are mounted by bearings in the central openings of the end frames which permit free rotation of the rotor shaft about its longitudinal axis. Presently, the bearings include many parts and require substantial time to assemble and install in the end frames.
The inner raceways of the bearings held in the central openings of the end frames are typically fixed to the rotor shaft at predetermined locations. Thus, the relative location of the end frames is determined by the predetermined locations on the rotor shaft. The presence of tolerances in the dimensions of the rotor shaft, the end frames and the stator core occasionally results in the stator core and end frames not fitting together as they should in the assembly of the machine. A minor misalignment or structural irregularity of the rotor shaft may cause the shaft to wobble as it rotates. The wobble causes variations in the air gap (i.e., the distance separating the rotor and the stator core) which results in undesirable noise and vibration.
Another aspect of the assembly of the electric motor which is labor intensive is the electrical connection of the windings to a plug and terminal assembly used to connect the windings to a source of electricity and to control operation windings for starting the machine. Presently, there are at least four connections used to electrically connect the terminal end of each magnet wire to the plug and terminal assembly. The magnet wire is first connected to a terminal having sharp ridges which pierce the insulation on the wires to make electrical contact as the terminals are crimped against the magnet wire. The ridged terminal is connected to wire having plastic insulation, which is in turn connected to a terminal on the plug and terminal assembly. The terminal on the plug and terminal assembly is connected to the circuitry in the plug and terminal assembly. Typically, only two of these connections are made during assembly of the motor. However, each terminal connection is a more likely site for failure. Moreover, connection of the plug and terminal assembly to the end frames of the motor presently requires separate fasteners. The use of such fasteners, or alternative joining methods such as welding or soldering, adds the cost of the fasteners or joining material, and the cost of labor to connect the plug and terminal assembly by application of the fasteners or joining material.
In order to ground the motor end frames, a separate assembly step is required for ground connection. For instance, a screw may be received through an end frame and into the plug and terminal assembly, or the connection may be by insulated wire. The insulated wire is connected to the end frame by a screw or a clip, which are additional materials which require additional time to manipulate during assembly of the motor.